The present invention relates to an optical memory element capable for at least one of the recording, reproducing and erasing operations by means of optical beam irradiation, or more specifically to an optical memory element substrate and a photo-mask for use in transcribing guide tracks and track address patterns onto an optical memory element substrate.
Of late, demand for optical memory elements is increasing year by year due to their high density and large capacity. Optical memory elements can be classified into the three types in terms of their use: read-only memory, add-on memory and erasable memory.
Optical memory elements ensure high density and large capacity because a bit (information recording unit), which is determined solely by optical beam diameter, can be made as small as about 1.mu.m in size. This fact, however, causes a number of restrictions for an optical memory system; optical beam positioning must be extremely accurate in order to record information precisely in a predetermined point or reproduce proper information recorded in a preselected point. In the case of read-only type optical memory, in general, address data can be recorded together with data information so that it is possible to properly position the optical beam while recorded data information is being reproduced. For add-on memory or erasable memory type, on the other hand, it is difficult to record address data together with data information in the memory. Therefore, in the add-on memory or the erasable memory, guide signals or guide addresses are normally recorded on the memory substrate.
For instance, an optical memory element to be used as add-on or erasable memory normally has guide tracks in the substrate, to guide an information-recording or information-reproducing beam spot to a specified position on the optical memory element. In many cases, track address-indicating data is written in a part of each guide track to locate the guide track.
FIG. 1 shows the essential part of the memory substrate of the add-on or erasable memory type in perspective view. As shown, stripe-shaped grooves are formed in the substrate, and information is recorded or reproduced along the grooves. Though not shown, the grooves are intermittent in the circumferential direction so as to provide address bit information for each groove. In an optical memory element of a disc shape as shown in FIG. 2, in particular, guide track 3 and tracks address portion 2 (together with sector addresses if the tracks are divided into many sectors) are formed concentrically or spirally in the substrate 1. For the purpose of simplicity, only one track 3 and only one address portion 2 are shown in FIG. 2.
A number of methods have been practiced for manufacturing a disc substrate provided with tracks and addresses as described above. A mask which carries guide tracks and addresses thereon may be airtightly placed on a resist film applied on a glass disc, to form the tracks and addresses in form of grooves or pits directly on the glass disc. In this method, it is essential to form the guide tracks in such a manner that the guide track center coincides with the center hole of the glass disc as precisely as possible. With poor concentricity between the guide tracks and the glass disc, when the glass disc is rotated for recording, reproduction or erasing with the center hole fixed on a rotary shaft 4, as shown in FIG. 3, the guide tracks vibrate significantly with respect to a recording, reproducing or erasing laser beam 5, and hampering track servo operation for controlling the lens position. For the above reason, it is necessary to join the guide track center and the glass disc center hole as precisely as possible (with deviation allowance preferably within 20.mu.m) in the optical exposure of the guide tracks.
The above manufacturing method can cause another problem in the guide track and track address pattern transcription process. The resist film 6, when applied, tends to have a partial rise 8 at the peripheral portion of the disc glass as shown in FIG. 4. This rise 8 often impairs close contact between the resist film 6 and the photo-mask 7, although it is desirable that they are made in close contact with each other. The width of this deficient close contact is usually 1-2mm. In FIG. 4, the guide track and address pattern on the photo mask 7 is omitted. The deficiency in close contact prevents the guide tracks and track addresses from being formed at proper positions.